“We showed with little doubt that oil consumed by marine bacteria did reach the larger zooplankton that form the base of the food chain,” Monty Graham of Dauphin Island Sea Lab and University of South Alabama told environmentalresearchweb. “These zooplankton are an incredibly important food-source for many species of fish, jellyfish and whales.”

The Gulf of Mexico is rich in wildlife and home to two species of whale – Bryde’s whale and the sperm whale, which is endangered. Fishing, both commercial and recreational, is one of the largest industries in the region; according to the US National Oceanic and Atmospheric Administration, commercial fishermen earned $659 million in total landings revenue in 2008, with red snapper and shrimp two of the most important species.

Some microbes can consume hydrocarbons, such as oil and methane. Once these microbes are eaten by zooplankton the oil-based carbon enters the food web. Graham and colleagues were able to track this because carbon from hydrocarbon sources tends to contain a relatively low proportion of the carbon-13 isotope. The researchers found that the amount of carbon-13 in plankton did, indeed, decrease following the arrival of surface oil spills.

The team analysed two groups of plankton: those between 1 micron and 0.2 mm in size, the “small suspended particulate” fraction; and those between 0.2 and 2 mm, or “mesozooplankton”. It is likely that the smaller group provides a food source for their larger neighbours.

The sampling region was more than 100 km north of the Deepwater Horizon well head, in an area that saw three northwards pulses of surface oil. Two of these spills took place before the successful capping of the well on 15 July. Samples were taken at four sites on the middle and inner continental shelf, generally from one metre below the surface and one metre above the sea-floor, and at two reference locations in Mobile Bay.

The team found that the ratio of carbon-13 in the plankton samples from the middle and inner shelf decreased during the two initial slicks, indicating that they had absorbed carbon from the oil. It took 2 to 4 weeks for the ratios to return to normal. Similarly, the sites furthest offshore also saw a change in carbon ratio in mid-August, following the pulse of residual oil in late-July.

These changes occurred for both size groups of plankton, and for both the top and bottom of the water column.

“These data provide strong evidence that labile fractions of the oil extended throughout the shallow water column during northward slick transport and that this carbon was processed relatively quickly at least two trophic levels beyond prokaryotic consumers,” write the researchers in ERL.

From 22 April 2010 the Deepwater Horizon spill released around 780,000 cubic metres of Sweet Louisiana crude and 205,000 mT of methane for a period of 85 days. Roughly 25% of this was directly recovered or burnt at sea, leaving the rest to be degraded naturally or by chemical dispersants.

“The continuing search for where the oil went should not only include direct evidence of existing pools of oil, but also the shadows of where the oil once was as indicated by so-called "biomarkers" – such as light carbon isotopes in the bodies of plankton,” said Graham.

Now Graham and colleagues will continue lab and field work to understand “the fate and transport of petroleum hydrocarbons in marine planktonic ecosystems under the influence of natural or human-mediated chemical dispersion”.

Deepwater Horizon Oil Spill: ERL Call for Papers. Environmental Research Letters (ERL) – environmentalresearchweb's sister product – would like to invite you to submit your research into the effects of the Deepwater Gulf oil spill to a dedicated focus issue. This issue will build a collection of research on the initial impacts of the disaster and is the perfect forum to present your work to a massive global audience.